BIOL 2500 - Topic 6 (part 5)

Two-point test crosses use

It helps calculates the recombination frequency between two linked genes, which we can then use to infer the distance between the genes

Two-point test cross disadvantage

It is not the most effective way to build a map

Three-point test cross

It is a test cross designed to identify genetic linkage between 3 genes and helps determine recombination frequencies between linked genes, which helps with determining gene order

Three-point test crosses can be used for…

It can be used with autosomal genes, as well as sex-linked genes, using trihybrid females and hemizygous males

Resulting genotypes in three-point test crosses

It results in 8 genetically different genotypes from each parent, 2 being parental and 6 being recombinant

Recombinant genotypes in a three-point test cross

4 single crossovers and 2 double crossovers

Single crossovers in three-point test crosses

2 are between genes a and b and then the other 2 are between b and c

Genotypic ratio of three-point test crosses

You get independent assortment? But the the genotypic frequencies are unequal (meaning it is incomplete linkage)

How can you identify the parentals?

They are the two that are the most observed

How can you identify double crossed recombinants

They are the two that are least observed

Null hypothesis for three-point test crosses

The genes are unlinked

Expected number of phenotypes in three-point test crosses

You expect a 1:1:1:1 ratio (meaning it is unlinked)

SCO vs. DCO

SCO —> Single cross over

DCO —> Double cross over

How do we know which gene is in the middle?

Compare the parentals with the DCO recombinants, such that all of the genes should be the same except for one, which is your middle gene

Regular recombination frequency formula

(Number of recombinants / Total observed number) x 100

Calculating the RF for the two genes at the ends

(ALL of the recombinants / Total observed number) x 100

Why are their less DCOs then SCOs

Because the probability of two crossovers happening right next to each other is small

Interference

It refers to the fact that the observed number of DCOs relative to the expected number is reduced, if two 2 SCO events happen independently

What does interference indicate?

It indicates the influence of certain processes that can limit the number of crossovers that can occur in short length chromosomes

Interference value essentially compares…

It essentially compares the frequency of observed DCO events with the expected number of DCOs

Expected DCO (formula)

(RF₁) x (RF₂) x Total number of progeny

Coefficient of coincidence (COC)

Observed DCO / Expected DCO

Interference formula

1 - COC

Positive Interference number

1.) It represents the proportion of DCO that was expected but not produced in the experiment

2.) If I = 0.40, then the number of DCO’s was 40% lower than expected

Negative Interference number

This is when we get more DCOs produced than expected

When is Interference = 0

When observed DCOs = Expected DCOs